Method for eliminating bismuth from molten lead by adding calcium-magnesium alloys

ABSTRACT

The invention concerns a method for eliminating bismuth from molten lead with magnesium and calcium, which consists in inputting magnesium and calcium in the form of lumps of two Mg—Ca alloys, one containing predominantly Mg, the other containing predominantly Ca, each having a Liquidus temperature lower than 650° C., and preferably less than 600° C. The Liquidus temperatures of the two alloys are preferably close to less than about 20° C. A particularly efficient treatment is obtained with a Mg—Ca alloy close to the eutectic alloy at 16.2 wt. % of calcium and containing 12 to 25% of calcium and a Ca—Mg alloy containing between 60 and 80% of calcium. The invention enables fast elimination of bismuth and adjustment of the Ca/Mg ratio during treatment.

FIELD OF THE INVENTION

[0001] The invention relates to a molten lead refining method toeliminate bismuth from said molten lead, using calcium and magnesium.

STATE OF THE RELATED ART

[0002] The treatment of lead with calcium in order to eliminate bismuthhas been known since 1917 and has, since that time, made it possible toreduce the bismuth content to less than 0.05%. Guillaume Kroll's U.S.Pat. No. 1,428,041, filed in 1920, already described the combined use ofcalcium and magnesium for this purpose and subsequently gave rise to theKroll-Betterton method, which is still widely used at the present time.It consists of simultaneously adding calcium in lump form and magnesiumin ingot form. Experience has shown that the most effective proportionof these two refining agents is approximately ⅓ of calcium to ⅔ ofmagnesium.

[0003] The main problem encountered when introducing these metals intothe lead bath lies in the significant difference in their density (1.5for Ca and 1.7 for Mg) with respect to that of lead (10.5), which tendsto keep them on the surface of the bath and induces significant lossesdue to air oxidation.

[0004] One means to decrease the oxidation of calcium and magnesium isto use Mg—Ca alloys. In 1938, the patent U.S. Pat. No. 2,129,445(American Metal Company) indicated this possibility and particularlydescribed an alloy containing 79.4% magnesium and 20.6% calcium. Thepatent EP 343012 (Timminco) covers the use, for this application, of anMg—Ca alloy containing 65 to 75% magnesium, and the addition of thisalloy to the lead bath at a temperature between 415 and 500° C., suchthat the alloy dissolves without melting. The disadvantage of using analloy of the given composition is that it is not possible to adjust theproportion of reagents during treatment. However, if it is desired tomake this adjustment, it is necessary to use pure calcium or magnesiumin addition to the alloy.

[0005] Another solution to prevent oxidation consists of injectingadditives in powder form. The Fench Patent FR 2514786 (Extramet)describes the introduction of calcium and magnesium in the form of amixture of Ca—Mg alloy granules, by injection via an inert carrier gas.The granules are, preferentially, a mixture of granules of both Mg—Caphase diagram eutectic alloys (alloys containing 82% and 16.2% Ca). Thepatent application WO 98/59082 filed by the applicant describes thetreatment of lead with a cored wire with a meltable coating containing amixture of calcium and magnesium powders. The introduction of reagentsin the form of a mixture of powders by injection or the cored wiretechnique does not allow the adjustment of their respective proportionduring treatment.

[0006] In this way, the aim of the invention is to enable effectiveelimination of bismuth from lead using calcium and magnesium in lowoxidation forms, while retaining the possibility to adjust the Ca/Mgratio during treatment.

PURPOSE OF THE INVENTION

[0007] The invention relates to a method to eliminate bismuth from leadby means of magnesium and calcium, wherein the magnesium and calcium areintroduced in the form of lumps of two Mg—Ca alloys, one predominantlyMg and the other predominantly Ca, each having a liquidus point below650° C., and preferentially 600° C. The liquidus points of the twoalloys are preferentially within less than 20° C. of each other. Aparticularly effective treatment is obtained with an Mg—Ca alloy similarto the eutectic with 16.2% (by weight) of calcium and containing 12 to25% of calcium, and a Ca—Mg alloy between 60 and 80% of calcium.

DESCRIPTION OF THE INVENTION

[0008] Trying to combine the advantages of the use of Mg—Ca alloys,essentially an improved oxidation resistance, and that of the use of twoseparate reagents, i.e. the possibility of adjusting the Ca/Mg ratioduring treatment, the applicant first of all had the idea of selectinglumps of alloys of the same composition as those described in FR2514786, i.e. eutectic compositions containing 16.2 and 82% calcium. Thetest carried out with these alloys and with alloys of differentcompositions first of all demonstrated that it was possible to select acertain composition range both for the calcium-poor alloy and for thecalcium-rich alloy, provided that the liquidus point remains below 650°C., which is equivalent to less than 30% calcium for the calcium-pooralloy, and a range of 60 to 90% for the calcium-rich alloy. The liquiduspoint is, preferentially, less than 600° C., or approximately 8 to 25%of calcium for the poor alloy and 67 to 87% for the rich alloy. In fact,the advantage of the alloys is barely noticeable if the calcium ormagnesium content becomes too low. In this way, the ranges of effectivecontents are 12 to 25% for the poor alloy and 60 to 80% for the richalloy. To obtain comparable behaviour for both alloys, it is preferableto select alloys with a close liquidus point, typically with adifference of less than 20° C.

[0009] For the calcium-poor alloy, it is possible to select in thevicinity of the eutectic containing 16.2%, wherein the liquidus point is516° C. This liquidus point corresponds, for the rich alloy, toapproximately 75% calcium. When the lead treatment is carried out byimmersing in a metal cage, the pair of alloys defined is well suited,and it is of no interest to deviate too much from it so as not toperform the treatment at an excessively high temperature. It is possibleto define an optimal content range of 12 to 20% Ca for the poor alloyand 70 to 77% for the rich alloy.

[0010] In addition, the experiments conducted by the applicantdemonstrated that the eutectic alloy containing 82% calcium showed amarked tendency to ignite in air, that this tendency decreased with thecalcium content, but above all that it disappeared suddenly andunexpectedly for a calcium content of approximately 67%. When the alloysare introduced into the lead in a vortex, this tendency to ignite in airis more of a hindrance, and it is preferable to select a content between60 and 67% calcium for the rich alloy, resulting in the acceptance of aliquidus point between 600 and 650° C. In this case, it is of interestto deviate slightly from the eutectic composition for the poor alloy, soas not to have an excessively deviant liquidus point, the optimalcalcium content range being between 20 and 25%.

[0011] It is advantageous for the user that these two alloys come iningots of different sizes or shapes, so as to recognise them easily. Theunit size must be appropriate to obtain the desired proportion betweenthe calcium and magnesium, of the order of ⅓ calcium, easily, and tomake the necessary corrections during treatment.

[0012] Unlike the disclosure in patent EP 343012 mentioned above, it isobserved that these alloys with a lower melting point than alloyscontaining 30% calcium, which melt in the lead before dissolving,dissolve more quickly, and result in more rapid refining. In addition,contrary to what may have been expected, the simultaneous introductionof two meltable alloys, wherein the average composition corresponds to anon-meltable alloy, does not result in the formation of a solid mass.

EXAMPLES Example 1

[0013] A bath containing 13 kg of molten lead at 480° C., in which 0.2%of bismuth was introduced, was prepared. 44.1 g of calcium lumps and95.9 g of magnesium ingots cut with a metal saw were added. The reagentswere immersed in the lead using a steel cage. The bath was maintained at480° C. for 4 hours. The temperature was then lowered to 330° C. for 1hour. The bath was analysed every half hour. The bismuth content changedfrom 0.2% at the start of the treatment to 0.1% at the end of the 480°C. maintenance period and to 0.01% at the end of the cooling period at330° C.

Example 2

[0014] The same quantity of calcium and magnesium as in example 1 wasintroduced, but in the form of 140 g of alloy containing, by weight, 33%calcium and 67% magnesium. The bismuth content changed from 0.2% at thestart of the treatment to 0.1% at the end of the 480° C. maintenanceperiod and to 0.01% after one hour of cooling at 330° C.

Example 3

[0015] The same quantity of calcium and magnesium as in example 1 wasintroduced, but in the form of 102 g of alloy containing 19% calcium and38 g of an alloy containing 65% magnesium. The bismuth content changedfrom 0.2% at the start of the treatment to 0.1% after being maintainedfor one half-hour at 480° C. and then to approximately 0.06% after 4hours, and finally to 0.01% after one hour of cooling at 330° C.

Example 4

[0016] Example 1 was reproduced by limiting the 480° C. maintenanceperiod to 2 hours. The bismuth content was 0.15% at the end of saidmaintenance, and 0.05% after one hour of subsequent cooling at 330° C.

Example 5

[0017] Example 2 was reproduced by limiting the 480° C. maintenanceperiod to 2 hours. The bismuth content was 0.1% at the end of saidmaintenance, and 0.05% after one hour of subsequent cooling at 330° C.,as in example 4.

Example 6

[0018] Example 3 was reproduced by limiting the 480° C. maintenanceperiod to 2 hours. The bismuth content was 0.1% at the end of saidmaintenance, but had reached 0.02% after one hour of subsequent coolingat 330° C. Therefore, it is observed that the simultaneous addition oftwo alloys according to the invention makes it possible to obtain, withrespect to the prior art, an improved bismuth elimination rate for thesame treatment time.

1. Method to eliminate bismuth from lead using magnesium and calcium,characterised in that the magnesium and calcium are introduced in theform of lumps of two Mg—Ca alloys, one predominantly Mg and the otherpredominantly Ca, each having a liquidus point below 650° C., andpreferentially 600° C., and in that the liquidus points of the twoalloys are within less than 20° C. of each other.
 2. Method according toany of claim 1, characterised in that the Mg—Ca alloy contains 12 to 25%of calcium, and the Ca—Mg alloy 60 to 80% Ca.
 3. Method according to anyof claims 1 or 2, characterised in that the alloy lumps are introducedinto the bath inside a cage.
 4. Method according to claim 3,characterised in that the Mg—Ca alloy contains 12 to 20% Ca, and theCa—Mg alloy 70 to 77% Ca.
 5. Method according to any of claims 1 or 2,characterised in that the alloy lumps are introduced into the bath usinga vortex.
 6. Method according to claim 5, characterised in that theMg—Ca alloy contains 20 to 25% Ca, and the Ca—Mg alloy 60 to 67% Ca. 7.Method according to any of claims 1 to 6, characterised in that thelumps of each of the alloys are of different shapes or sizes.